The tricky part of painting iron is that it often has a form and shape that makes it difficult to reach certain parts properly – such as fencing, pipes or radiators. Preparatory work is however the same as for most other materials: clean, scrape and sand.
Check whether the iron has been treated for rust-prevention in some way – an invisible film of oil can be impossible to paint on. In such case, clean with a degreasing agent and rinse carefully. The scraper used to remove old loose paint should have a hard-metal blade – they keep their sharpness considerably better than steel blades.